Radio frequency power amplifiers are commonly used in various applications. Examples of such applications include, broadcasting, satellite and cellular communications. Indeed, some cellular communications systems include video capability. In a radio frequency system, the information signals are modulated onto a carrier frequency. A power amplifier is generally used to amplify the high frequency carrier signal
The high frequency carrier signal component commonly used in a cellular system includes a fundamental frequency of few hundred MHz to a few gigahertz. Depending on the application, the bandwidth of the modulating signal component can be from 20 megahertz, such as for multi-carrier W-CDMA applications to 30 kilohertz, such as for cellular communications. Radio frequency bandwidth generally includes the fundamental frequency Second and third harmonics are also possible.
Generally, the presence of higher harmonics is undesirable because they waste power and can be outside the desired bandwidth, where they may interfere with other radio frequency signals.
A radio frequency power amplifier is typically constructed using a printed circuit board, with various components of the radio frequency power amplifier circuit installed on the printed circuit board. The radio frequency power amplifier circuit typically includes an input, an active element, a bias circuit element, an output matching network, and an output. The active element may include an operation amplifier, a power transistor, or another suitable active element known in the art. In operation, an input signal, for example, a video signal or a cellular communications signal, is coupled to the amplifier input, amplified, and the amplified signal is output for broadcast, such as via an antenna.
In most applications a desirable characteristic for power amplifier circuits generally include adequate linearity and efficiency. Linearity is important because the more linear the characteristic of the amplifier, the less signal distortion is introduced as a consequence of amplification. Linearity is generally measured over a band of frequencies.
Conversely, there are circuit elements for which the impedance should be low, or which should vary with frequency in specified ways. As will be discussed herein, certain components within a radio frequency power amplifier circuit require impedances with certain values and/or which vary for different frequencies. For example, to avoid perturbations in a direct current bias voltage within the amplifier circuit, the impedance at the video frequency should be carefully controlled.
In addition, relative physical dimensions of circuit elements and connections within radio frequency power amplifier circuit are an important consideration with respect to the wavelengths of the radio frequency signals to be amplified. The high frequency (and consequently, the short wavelength) of radio frequency signals can lead to significant phase differences appearing in signals, for example, between the opposite ends of a bias feed portion of the radio frequency amplifier circuit. Such phase differences are corrected with the use of appropriate impedance matching.
Additionally, where an impedance mismatch exists between a signal source and a signal destination, according to transmission line theory, the signal may be reflected back along the transmission line connection toward the source. This reflection leads to undesirable signal reflection loss.
In design applications which require both high frequency and high power, it is a challenging task to combine large active transistors with other needed elements. This is particularly true in a radio frequency power amplifier design. Improper combining of such elements causes a multitude of problems, such as unbalanced signal phase and amplitude, multimode oscillations, use of higher loss matching elements, and thermal and electrical memory effects due to improper impedance matching. Unbalanced signal phase and amplitude result in lower realizable gain and degraded gain flatness. Multimode oscillations lead to instabilities in the power amplifier, as well as undesirable out of band signal emissions. In addition, the use of higher loss matching elements leads to appreciable dielectric and metal losses. Lastly, thermal and electrical memory effects due to improper impedance matching results in slower realizable switching speeds, degraded AM-AM, and AM-PM distortion products.
Several approaches to providing DC supply to radio frequency power amplifier design have been evaluated. A first approach includes the use of a quarterwave, i.e., a quarter-wavelength, transmission line. Such a quarterwave solution suffers from high inductance at video frequencies, The high inductance leads to undesirable lower video bandwidth and higher distortion. The high second harmonic impedance leads to higher distortion. The uneven impedance across active cells leads to undesirable thermal imbalance, instability, uneven power, and/or lower gain.
A second approach to providing a radio frequency power amplifier design includes use of the quarterwave transmission line of the first approach, in addition to the use of a parallel tank. Tank circuits provide parasitic elements via the use of bypass capacitors. In this second approach, a bypass capacitor is coupled in parallel to the quarterwave transmission line. Such a parallel tank approach still suffers from undesirably high modulation impedance, undesirably high second harmonic impedance, parasitic loading, lower radio frequency bandwidth, uneven impedance across active cells, and multi-mode oscillations. Further, parasitic loading results in undesirably higher losses.
A third approach makes use of the quarterwave transmission line with electrical field coupling via the use of a dielectric member. Such electrical field coupling suffers from uncontrollable variations and still provides undesirably high modulation impedance, high second harmonic impedance, uneven impedance across cells, and multi-mode oscillations.
As such, it is desirable to provide a radio frequency power amplifier design that substantially mitigates uncontrollable variations in electrical field coupling, modulation impedance, second harmonic impedance, uneven impedance across cells, and multi-mode oscillations.
Cellular communications systems utilize a plurality of radio frequency transceivers to relay information between a wired (or fiber optic) communications network and a plurality of mobile telephones. Each radio frequency transceiver is associated with an antenna, which is typically located upon a tower. The communication range of the transceiver defines a cell.
Such radio frequency power amplifiers receive as an input thereto a low power radio frequency signal. The radio frequency signal may contain voice, data, video or any other desired information. Because of its low power, this signal is not suitable for transmission over the distances commonly encountered in cellular communications systems. Therefore, the low power radio frequency signal must be boosted in power or amplified, to obtain the desired communication range of the cell.
Although such contemporary radio frequency power amplifiers have proven generally suitable for their intended purposes, they possess inherent deficiencies that detract from their overall effectiveness and desirability. For example, the construction of transceiver stations, typically accompanied by a tower upon which the antenna is mounted, is costly. By making the receivers of the transceivers more sensitive and by making the transmitters thereof more powerful, each transceiver can operate over a larger range and each cell can thus be larger. As the cell become larger, the system requires fewer cells. Consequently, this minimizes the cost of building additional transceiver stations.
One contemporary attempt to increase the power output of a radio frequency amplifier involves the use of larger amplifier transistors. However, limitations on the ability to fabricate larger amplifier transistors and the costs associated with the use thereof place limitations on their use in commercial equipment.
Another contemporary attempt to increase the power output of a radio frequency amplifier involves the use of multiple or ganged amplifier transistors. Two or more amplifier transistors are electrically connected in parallel. In this manner, the power outputs of the individual amplifier transistors ideally sum to provide the total power output of the radio frequency power amplifier.
However, when multiple amplifier transistors are utilized, undesirable distortions of the amplified radio frequency signal can result. These distortions of the amplified radio frequency signal can limit the gain of the radio frequency amplifier and can reduce the channel capacity of the radio frequency signals. Among these distortions are those associated with undesirable variations in electrical field coupling, modulation impedance, second harmonic impedance, third harmonic impedance, uneven impedance across cells, and multi-mode oscillations.
As such, although the prior art has recognized, to a limited extent, the problems associated with increasing the power output of transceiver stations, the proposed solutions have, to date, been ineffective in providing a satisfactory remedy. Therefore, it is desirable to provide transceivers having more powerful transmitters that do not suffer substantially from the undesirable effects of distortion of the amplified radio frequency signal.
Additionally, as the temperature of a radio frequency power amplifier changes, its operational characters also change. More particularly, as a radio frequency power amplifier gets hotter, the gain thereof typically decreases substantially.
Such temperature changes can be due to variations in ambient temperature and variations in load. Variations in ambient temperature cause variations in the operating temperature of the amplifier transistor because they vary the ability of the amplifier transistor to dump heat into its environment. The warmer the ambient temperature, the less effectively the amplifier transistor can transfer its own heat to the environment. Consequently, the warmer the ambient temperature, the warmer the amplifier transistor.
As greater demands are placed upon an amplifier transistor, it tends to operate at a higher temperature. An amplifier transistor which is amplifying a higher level input signal will tend to operate at a higher temperature than one which is amplifying a lower lever input signal or no input signal at all. This is because the transistor draws more source-drain current as the amplifier transistor amplifies a higher level input signal than is being drawn as the amplifier transistor amplifies a lower level input signal.
Since the gain of an amplifier transistor decreases as the temperature thereof increases, undesirable distortions of the amplified signal result from such temperature variations.
Thus, it is desirable to provide a radio frequency power amplifier that does not undesirably distort the radio frequency signal amplified thereby due to temperature variation thereof.